The present invention relates generally to wireless networks, and more particularly to handoff in wireless networks.
Code Division Multiple Access (CDMA) wireless networks are well known in the art for providing wireless voice and data services to mobile stations within the network's service area. CDMA is a spread spectrum technology, in that multiple users share the same broadband frequency range. Individual spreading codes, assigned to each of the multiple users, allows the signals for a particular user to be extracted from the transmitted signal. CDMA technology is well known in the art and will not be described in detail herein.
Mobile stations within a CDMA network communicate via base stations. Each base station serves a geographic area, often referred to as a cell. Each cell may be further divided into sectors, whereby each sector in a cell is served by one or more directional antennas associated with the base station serving the cell. Such a system is shown graphically in FIG. 1. FIG. 1 shows base station 102 with associated antenna system 104. The antenna system 104, as shown, contains three directional antennas 108, 110, 112, each serving sectors A, B, C of cell 106 respectively. One skilled in the art will recognize that while FIG. 1 shows one antenna serving each of the sectors, base station 102 may also contain multiple antennas for each sector.
Each sector of a cell in a CDMA wireless network is associated with a pilot signal. As will be described in further detail below, one of the uses of pilot signals is to facilitate handoff of the mobile station from one sector to another sector. The base stations of a CDMA network continuously transmit a pilot signal on each of its sectors. The pilot signals all have the same spreading code, but with a different code phase offset. This phase offset allows the pilot signals to be distinguished from one another. These phase offsets are referred to as PN offsets, and are generally in the range of 0 to 511. CDMA networks generally use a subset of the available offsets, usually in increments of 2, 3 or 4, for a total of 256, 170 or 128 unique PN offsets, respectively.
Most CDMA networks have at least several hundred 3-sector cells. Since each sector requires its own PN offset, the PN offsets in the network are not unique, and must be shared among the sectors in the network. In order to ensure unambiguous handoffs, PN offset reuse planning techniques are utilized to ensure that sectors having the same PN offsets are far enough apart so that PN offsets are locally unique. This ensures that a mobile station at any one location within the CDMA network is unlikely to detect the same PN offset from multiple sectors.
Mobile stations use the PN offsets for handoff as follows. Referring to FIG. 1, assume that mobile station 114 in sector A of cell 106 is being served by antenna 108, and is traveling in the direction of arrow 116, toward cell 124 with base station 120. As mobile station 114 gets closer to base station 120, it will detect the pilot signal being transmitted by base station 120 over antenna 122 in sector C of cell 124. More particularly, the mobile station 114 detects the pilot signal using a component generally referred to as a searcher. The searcher within a mobile station continuously checks for the appearance of new pilot signals and for the disappearance (or weakening) of previously detected pilot signals. When either of these events occurs, the mobile station transmits pilot signal information to the CDMA network. In particular, as shown in FIG. 1, the mobile station 114 will transmit this information to the network controller 126, which is responsible for mobility management within the network. In an IS-95/IS2000 compliant wireless network, these messages are called Pilot Measurement Strength Messages (PSMM). In 1xEV networks conforming to the IS-856 standard, these messages are called Route Update Messages (RUM).
In FIG. 1, as the mobile station 114 moves toward cell 124, the signal strength from the pilot signal being transmitted by antenna 122 in sector C of cell 124 will increase in strength. In response to a message with this information, the network controller 126 will allocate a new traffic channel in sector C of cell 124 (over antenna 122) so that the mobile station 114 may also communicate with base station 120 via this new traffic channel. As the signal strength from the pilot signal being transmitted by antenna 108 in sector A of cell 106 weakens, the network controller 126 will de-allocate the traffic channel in sector A of cell 106. This type of handoff is called a soft handoff, because the new traffic channel is allocated prior to de-allocation of the existing traffic channel. One skilled in the art will recognize that the particular steps and protocols for performing mobility management (e.g., handoffs) in a CDMA system are defined in the relevant network standards (e.g., IS-95/IS-2000, 1xEV/IS-857, etc.), and the handoff described herein is a generalized description.
In order to further facilitate handoff in CDMA systems, the network maintains neighbor lists associated with each of the sectors in the network. Each neighbor list contains an identification (i.e., PN offset) of the pilot signals in the neighboring sectors that mobile stations are likely to detect while communicating within a particular sector. For example, with reference to FIG. 1, the neighbor list for sector A in cell 106 would contain the PN offset of the pilot signal of sector C of cell 124 (transmitted by antenna 122) because a mobile station in sector A of cell 106 is likely to detect the pilot signal being transmitted in sector C of cell 124. The neighbor list also identifies the particular sector and cell associated with each of its identified pilot signals.
The neighbor lists associated with the serving sector(s) are also sent to the mobile stations after each handoff (and in certain cases after initial communication with the network) via a Neighbor List message. The mobile stations use the neighbor lists to focus the operation of the searcher element to search for the pilot signals of the most likely handoff candidate sectors.
Neighbor lists are generally populated and maintained by network operators. At the time of initialization of the network, the network operator manually populates the neighbor lists. This initial population of neighbor lists is complex. Each sector can generally detect one or two sectors from each of the closest four or five cells. Further, special RF propagation conditions (e.g., “ducting” along city streets with high-rise buildings and over-the-water propagation effects) may also require distant sectors to be included in the neighbor lists. Such propagation conditions are not always obvious to network planners. Further, extensive drive testing is usually needed for proper initial population of neighbor lists. Such drive testing is difficult, time consuming, and prone to human errors. Further, the constant evolution (e.g., adding, removing and changing equipment) of CDMA networks adds to the complications involved in maintaining up-to-date neighbor lists. In view of these difficulties, various automated tools exist for populating and maintaining neighbor lists. However, even with such automated tools, current CDMA networks often contain errors in their neighbor lists.
One possible error in a neighbor list is identification of a pilot signal associated with a sector that is not actually a neighbor. Such listing may have some impact on network performance, as it may result in the mobile stations spending time searching for a pilot signal that does not exist. While undesirable, the identification of a pilot signal that is not actually a neighbor is a relatively minor problem.
The more significant problem arises when a pilot signal that is present in a sector, and therefore should be identified on the sector's neighbor list, is omitted from the neighbor list. If a pilot signal is not identified on a current neighbor list of the mobile station, then the sector associated with that pilot signal is not a handoff candidate, and the mobile station cannot engage in communication over the traffic channel of that sector. Due to the nature of CDMA communications, the signals from the non-identified sector interfere with the downlink signals to the mobile station. Further, the signals from the mobile station interfere with the uplink signals in the non-identified sector. This interference results in reduced network capacity. In severe cases, where the unidentified pilot signal is very strong, this interference may result in dropped calls.